Sometimes opening a big PDF file with your favourite image editor (maybe GIMP?) only to extract a JPG of the first page is a waste of time.
Using the convert command (from the ImageMagick suite) you can do this task in less time needed to launch the image editor itself (without doing the job)… 🙂
convert -define pdf:use-cropbox=true -density 72 -quality 80 -resize 768x1024! "filename.pdf" firstpage.jpg
The trick here is to put a [pagenumber] after the PDF file name! Very clever… 😉